Cave
A cave is a natural underground space large enough for a human to enter. Caves form naturally by the weathering of rock and often extend deep underground. The word "cave" can also refer to much smaller openings such as sea caves, rock shelters, and grottos. Formation The formation and development of caves is known as speleogenesis. Caves are formed by various geologic processes and can be variable sizes. These may involve a combination of chemical processes, erosion from water, tectonic forces, microorganisms, pressure, and atmospheric influences. Types Solutional Cave Solutional caves are the most frequently occurring caves and such caves form in rock that is soluble, such as limestone, but can also form in other rocks, including chalk, dolomite, marble, salt, and gypsum. Rock is dissolved by natural acid in groundwater that seeps through bedding-planes, faults, joints and so on. Over geological epochs cracks expand to become caves and cave systems. The largest and most abundant solutional caves are located in limestone. Limestone dissolves under the action of rainwater and groundwater charged with H2CO3 (carbonic acid) and naturally occurring organic acids. The dissolution process produces a distinctive landform known as karst, characterized by sinkholes, and underground drainage. Limestone caves are often adorned with calcium carbonate formations produced through slow precipitation. These include flowstones, stalactites, stalagmites, helictites, soda straws and columns. These secondary mineral deposits in caves are called speleothems. Primary Cave Some caves are formed at the same time as the surrounding rock. These are called primary caves. Lava tubes are formed through volcanic activity and are the most common 'primary' caves. The lava flows downhill and the surface cools and solidifies. The hotter lava continues to flow under that crust, and if most of the liquid lava beneath the crust flows out, a hollow tube remains, thus forming a cavity. Examples of such caves can be found on the Canary Islands, Jeju-do and other places. Kazumura Cave near Hilo, Hawaii is a remarkably long and deep lava tube; it is 65.6 km long (40.8 mi). Lava caves include but are not limited to lava tubes. Other caves formed through volcanic activity include rift caves, lava mold caves, open vertical volcanic conduits, and inflationary caves. Sea Cave Sea caves are found along coasts around the world. A special case is littoral caves, which are formed by wave action in zones of weakness in sea cliffs. Often these weaknesses are faults, but they may also be dykes or bedding-plane contacts. Some wave-cut caves are now above sea level because of later uplift. Elsewhere, in places such as Thailand's Phang Nga Bay, solutional caves have been flooded by the sea and are now subject to littoral erosion. Sea caves are generally around 5 to 50 metres (16 to 160 ft) in length, but may exceed 300 metres (980 ft). Glacier Cave Glacier caves occur in ice and under glaciers. They are formed by melting. They are also influenced by the very slow flow of the ice, which tends to close the caves again. Most glacier caves are started by water running through or under the glacier. This water often originates on the glacier’s surface through melting, entering the ice at a moulin and exiting at the glacier’s snout at base level. Heat transfer from the water can cause sufficient melting to create an air-filled cavity, sometimes aided by solifluction. Air movement can then assist enlargement through melting in summer and sublimation in winter. Ecology Cave-inhabiting animals are often categorized as troglobites (cave-limited species), troglophiles (species that can live their entire lives in caves, but also occur in other environments), trogloxenes (species that use caves, but cannot complete their life cycle fully in caves) and accidentals (animals not in one of the previous categories). Some authors use separate terminology for aquatic forms (for example, stygobites, stygophiles, and stygoxenes). Of these animals, the troglobites are perhaps the most unusual organisms. Troglobitic species often show a number of characteristics, termed troglomorphic, associated with their adaptation to subterranean life. These characteristics may include a loss of pigment (often resulting in a pale or white coloration), a loss of eyes (or at least of optical functionality), an elongation of appendages, and an enhancement of other senses (such as the ability to sense vibrations in water). Aquatic troglobites (or stygobites), such as the endangered Alabama cave shrimp, live in bodies of water found in caves and get nutrients from detritus washed into their caves and from the feces of bats and other cave inhabitants. Other aquatic troglobites include cave fish, and cave salamanders such as the olm and the Texas blind salamander. Category:Water Category:Minerals Category:Places